Electric circuit for supplying an X-ray tube with a predetermined anode current which is equal to the substantially constant saturation current of the tube



April 5, 1960 Filed Aug. 19, 1958 0 L. HOPPERMANN 2,931,909

ELECTRIC CIRCUIT FOR SUPPLYING AN X-RAY TUBE WITH A PREDETERMINED ANODECURRENT WHICH IS EQUAL TO THE SUBSTAN'I'IALLY CONSTANT SATURATIONCURRENT OF THE TUBE 2 Sheets-Sheet l cfiwenZ r 0/6 3432 woppermd m 8 4(a a (V J 72715 April 5, 1960 R SUPPLYING AN X-RAY TUBE WITH APREDETERMINED ANODE CURRENT WHICH IS EQUAL TO THE SUBSTANTIALLY CONSTANTSATURATION CURRENT OF THE TUBE Filed Aug. 19, 1958 2 Sheets-Sheet 2 R MIIIIIII 'IIIIIIIIII KT O (I) /V /V [e J28? d l opermenn flM LA /M m o.L. HOPPERMANN 2,931,909 ELECTRIC CIRCUIT F0 United States Patent CCELECTRIC CIRCUIT FOR SUPPLYING AN X-RA TUBE WITH A PREDETERMINED ANODECUR- RENT WHICH IS EQUAL TO THE SUBSTAN- .TIALLY CONSTANT SATURATIONCURRENT OF THE TUBE Ole Lutz Hoppermann, Charlottenlund, nearCopenhagen,

Denmark, assignor to Holger Andreasen, Copenhagen 8., Denmark, a Danishfirm Application August 19, 1958, Serial No. 755,998

Claims priority, application Denmark August 19, 1957 6 Claims. ((31.250-91 a special winding on one of the high voltage transformers whoseprimary windings across adjustable taps-are regulably connected to anautotransformer which is connected to a nominal line voltage through aregulating device for equalising variations in the said nominal linevoltage.

The present invention is'more particularly concerned with electriccircuits in X-ray apparatus for examination of coarse structures. Insuch apparatus the anode voltage of the tube which is decisive for'thehardness of radiation and the consequent power of'penetrationmust beadjustable over a suitable range ofvoltage. Furthermore, the anodecurre'nt'in the known apparatus of' the said kind can usually beregulated. In the known circuits of the aforesaid kind is thereforeusually provided a voltmeter for controlling the anode voltage of theX-ray tube and a milliammeter for controlling the anode current of thetube; in addition, there is a handle for adjusting the apparatus to agiven anode voltage by means of the adjustable taps, and another handlefor adjustment of Y The, use of an X-ray a predetermined anode current.installation with such a known electric circuit requires thusmanipulation of two handles and reading of two instruments.

It is the object of the present invention to devise an electric circuitin which there is only one instrument to be read, the above mentionedvoltmeter being dispensed with, and only one handle for, regulating theanode current, whereas the other handle is substituted by a switch thepositions of which are adjusted to a series of predetermined exact anodevoltages.

The present invention is basedupon the physicalfact that the emissionfrom an incandescent cathode .in accordance with Dushmanns formula: I i

V V v I.m=AT. where A and b are constants ofmaterial and T the absolutetemperature of the incandescent cathode, is solely dependent upon thesaid temperature and upon the material used. Under constant coolingconditions as in an X-ray apparatus the proportion between the electricenergy converted into X-rays and the electric energy supplied to thecathode is with substantial approximation constant. 9 I v i 'At an'anodecurrent predetermined for the type of the tube and which,-for example,may be five milliamps, the voltage across the incandescentcathode andconsequently across the primary voltage ofv the filament currenttransformer will be constant, andthis is utilised by 2,931,909 Pel -rethe present invention for measuring the anodevoltage of the X-ray tubeand the connected line voltage.

An essential feature of. an electric" circuit according to the inventionis that the primary winding or the filament current transformer or theprimaryjwinding of high voltage transformers having the special'windingis directly connected to permanent taps provided on the, autotransformerin such manner that the tube. has the predetera mined anode current whenthe autotransformer is ad justed to the nominal line voltage.

As a result, the voltmeter used in the known circuits, of the aforesaidkind is rendered superfluous. When the milliammeter isadjusted toindicate the predeterminedanode current which may be indicated by anindividual division on the graduated scale of themilliammeterfjtheconstant of the autotransformer for volts per windingwill be determined on the basis of the number of wind ings of thewinding to which the nominal voltage is con-. nected and the remainingtaps provided on the trans-r former will give off an exact voltagedetermined by the; number of windings between these taps, for examplealso the taps'for the high voltage transformers regulably con nected tothe autotransformer, and the said taps maythus f give off anode voltagesof predetermined values, an'cllth'e' individual positions of the handlefor the adjustable 'taps' for the andoe voltage can thus bedenoted bythe said exact values. V

v l twill be'appreciated that' if the line voltage during" the operationof the apparatus deviates from the nominal value the constant of theautotransformer'for volts perwinding will deviate from its normalvalueand the fila-, ment current transformer .will therefore not besupplied with the primary voltage corresponding to the predeter minedvalue of the anode current, wherefore the regulating device forequalising variations in the nominal line voltage has to be displaceduntil the milliammeter shows the predetermined anode current, and whenthis has taken place the autotransformer has the normal value for voltsper winding and the voltages across its different taps are of the exactvalues determined by the number of wind ings.

This indirect measurement of the anode voltage'of said voltagecorresponds to a great increase in the anode; current, which will beevident from Dushmanns'formula quoted above. It should be observed herethat experi-i ments have disclosed that a 1% increase in the voltageacross the incandescent cathode corresponds to'an increase of about 10%in anode current, that is, the anode current may be measured with anaccuracy ten times that obtained when measuring the voltage across the.incandescent cathode. Assuming that the milliammeter in the anodecircuit and the voltmeter used in the known apparatus for measuring theanode voltage are of identical precision, it will under the conditionsreferred as be ir'n possible, even by voltmeters which are of maximumprecision and provided with knife blade pointer and mirror scale, toadjust the said voltage as accurately as the anode current may beadjusted by'the milliammeter. i

In addition, however, also the anode voltage is measured withcorresponding accuracy owing to the proportionality between the voltageacross the incandescent cathode and the voltage across the taps for thehigh volt-' age transformer, and the said taps may thus be marked withvoltages corresponding to the respective numbers of windings, the loadon the high voltage transformers being always constant, for exampleequalto 5 milliamps.

- stereos The results obtained by the present invention are thus,compared with that obtained in the known circuit dia-' grams of theaforesaid kind;

I (1 Asubstantiallyv moreaccurateadjustment of fila ment voltage andanode voltage, v t

(2 A cost economy, since' the voltmeter is super fiuous and may bedispensed with, and

(3 That at anexposure the manipulation of an X-ray apparatus withanelectric circuitaccording to the inventionbecomes very simple.

In certain types of X-ray tubes the saturation current of, thetube ispractically independent of the anode voltage, and the present inventionmay, be'used directly in v connection with such X ray tubes. In othertypesv of tubes the saturation current increasesslightly andsubstantially proportionally with the anode voltage. It is, however,possible by simple measures to adjust the lastmentioned type of X-raytubes to electric circuits according to the present inventionbycompensating for thesaid increase in'the saturation current.

Inone embodiment of the electric circuitusing a de vice to compensatefor the increase, if any, in the saturation current of the X-ray tubethe device according to the invention consists of apermanentresistanceinserted into one of, the supply lines from thesystem voltage to theautotransformer and so dimensioned that the' voltage drop in theresistance compensates for the increase in the anode current whichincrease'is substantially proportional with the anode voltage. V

The result obtained is that by suitable dimensioning of the resistancethe voltage drop in same will produce a voltage drop in the primaryvoltage of the autotrans former and consequently a compensation againstthe said increase in the anode current sothat the latter remainssubstantially constant, even though.theautotransformer is adjusted tosupply a higher prirnarywoltage to the high voltage. transformers. I

In another embodiment of the electric circuit according to the inventionthe device consists of a permanent re-.

in the supply line to the said high voltage. transformer; produces acompensation against the increase. in the anode, current.

'Ina third embodiment of the .electric circuitaccording to. the.invention the deviceconsists of a second switch mechanically connectedto an anode voltage switch, the

' firstrnentioned switch being connected to the autotransformer in. suchmanner that the filament current decreases in predeterminedmanner atincreasing anode voltage.

I The result obtained is that by, suitable. placement of.

the connecting pointsof. the switches on the-autotransformer theincreased anode current originating from the increased primary voltagefor the high. voltage trans-. former is counteracted by a reduction ofthe filament current, so that the anode current remains substantiallyconstant and the anode voltage only becomes dependent on the position ofthe mechanically connected switches.

In a fourth embodiment of the circuit according. to the invention thedevice consists of a compensationtransfprrner whose primarywinding isconnected in parallel with the primary winding ofthe anode transformer,whereas its secondary winding is connected in series with andcounter-connected with. the primary windingtof the highvoltagetransformer havingthe special filament current winding.

The result obtained is that by suitable dimensioning of the ratio ofthe-compensation transformer the filament current may be reducedby anamount corresponding to the change of the anode voltage caused by aturning of 4 the second switch, so that the anode current despite thesaid change is kept constant.

In a fifth embodiment of the circuit according to the invention thedevice consists of a special transformer whose primary winding is.connected in parallel with the primary windings of the high voltagetransformer, whereas the secondary winding is in series with. andcounter-connected with the primary winding of the filament currenttransformer.

The result obtained is that at an increase in the primary voltage of thehigh voltage transformers the increased anode current in the'tube due tothe said increase may by suitable dimensioning of the ratio of thespecial transformer be compensated by a reduction of the filamentcurrent ofthe tube.

The invention will now be further described with reference to thedrawing, in which Figures 1-7 show various embodiments of an electriccircuit for supplying an X-ray tube according to the invention withcurrent.

In the embodiments of the electric circuit according t0 the invention asshown in Figures 1 and 2 the saturation currentof the tube is assumed tobe constant and of a predetermined value and that the anode current issubstantially independent ofthe anode voltage.

Figure l shows a first. embodiment of the electric circuit according tothe invention, comprising two high voltage transformers B and Creferredto as anode transformer and cathode transformer, respectively,and an independent filament current transformer D. A regulatortransformer A feeds the high voltage transformers B and C with primaryvoltage the value of which may be adjusted by means of a regulatingdevice E which may consist either of; a switch E sliding across a numberof taps provided on the regulating transformer A or of a contact brush Lwhich is adjustable over a stripped part of the surface of the windingsof the regulating transformer A, or of a combination of these. In thecase mentioned in the middle the regulating transformer A will usuallybealong the stripped part of. the surface of the windings oftheregulating transformer in case of dismantling of the;

apparatus during its adjustment in factory or workshop In the regulatingtransformer A are connected two poles N of apower line, thesaid'connection being estab fished-through a permanent tap O and bymeans of the contact brush K whichis displaceable along the strippedpart of the surface of the windings of the regulating transformer and sodimensioned that the regulation covers the expected deviations of theline voltage from the nominal voltage which the apparatus is intended towork at. The secondary windings of the high voltage transformers Band Care interconnected in series with a milliammeter m. Since the electriccircuit is assumed to be adapted for use in connection with an X raytube working at a definitemilliamp value the milliammeter need only haveone scale division corresponding to the said definite value. Thedivision may, however, have an indication in volts corresponding to thenominal line voltage. V e

The ratio of the filament current transformer D and the voltage fed tothe primary winding of the transformer, which voltage is given at thepermanent connecting points of the primary winding on the regulatingtransformer A, are so adapted that when the regulating transformer A-has the number ofvolts per winding for which it is intended the filamentwill have an emission corresponding to'the fixed anode current of theapparatus.

When the contact brush K is adjusted until the-milliammeter m indicatesthefixed anode current'oftheas mentioned above.

apparatus, the regulating transformer will receive the:

correct number of volts per winding, wherefore the regulatingtransformer A will be capable of feeding the high voltage transformers Band C with predetermined The adjusted in kilovolts and connected to aseries of taps on the regulating transformer A, or by means of a contactbrush L which is displaceable along the stripped winding surface of theregulating transformer A, in which case the regulating transformerusually will be ring wound. Finally, a combination as disclosed inFigure 1 may be used. In the latter case'adjustment of the anode voltagemay be continuous, and the scale of the regulating knob of the ringwound transformer may in that case be adjusted in kilovolts.

Figure 2 shows another embodiment of the electric circuit'according tothe'invention, comprising two high voltage transformers, an anodetransformer B and a cathode transformer F, respectively, the latterbeing, for example, a combined high voltage and filament currenttransformer.

The regulating transformer A feeds the anode transformer B and thecombined high voltage and filament current transformer F with primaryvoltage. The primary voltage of the anode transformer B may be adjustedby means of the regulating device E which may be designed An adaptationresistance M may be inserted in the filament current circuit if thefilament voltage of the tube R does not correspond to an integral numberof filament current windings of the combined high voltage and filamentcurrent transformer F. In the transformer F the ratio of the filamentcurrent winding and the primary voltage of the transformer, which isdetermined by the permanent connection points of its primary winding onthe regulating transformer A, adapted in such manner that when the lastmentioned transformer has the number of volts per winding for which itis calculated the emission of the filament will correspond to the fixedanode current of the apparatus. When the said anode current is obtained,the high voltage winding of the transformer F will furthermore produce apredetermined voltage so that the total high voltage across the anodecircuit of the tube has a fixed value. By adjustment of the contactbrush K which is displaceable along a stripped part of the surface ofthe windings of the regulating transformer A until the milliammeter mindicates the fixed anode current of the apparatus the regulatingtransformer A will be fed with the correct number of volts per winding,wherefore the regulating transformer will be capable of providing theanode transformer B with a predetermined primary voltage without thesaid voltage having to be measured by a voltmeter. The primary voltagescan be adjusted either by means of the regulating device E which may beadjusted in kilovolts or by means of the contact brush L as mentionedabove.

If the X-ray tube used in the electric circuit is such one in which theanode current increases at increasing anode voltage and if the electriccircuits disclosed above were used directly in connection with such atube the interval containing the permissible line voltages will bedisplaced upwards in case a low anode voltage is used, whereas the saidinterval will be displaced downwards in case a high anode voltage isused. Furthermore, with the use of such tubes it would be necessary tore-adjust the position of the contact brush K after each new adjustmentof the anode voltage.

In the embodiments of electric circuits in accordance with the inventionas described in the following a compensation for the increase in theanode current is obtained by an increase in the anode voltage.

Figure 3 shows a third embodiment of the electric circuit according tothe invention in which a resistance M of; the order of a few ohms isinsertedin the supply line from the one polelof the line voltage to theregulating transformers The strength of the current in the resistance Mdepends on the consumption of the X-ray tube and' is thus, since theapparatus involved is for adjustment of v a definite value of milliamp,solely dependent on thev On adjustment of an increased anode,

anode voltage. voltage the primary'winding of the combined filamentcurrent transformer and cathode transformer would owing to the increasedvoltage drop across the resistance receive a lower voltage, whereforethe temperature and 1 the emission of the filament will decrease so thatthe in- I crease in the anode current that would have occurred owing tothe increased anode voltage is equalised.

In the fourth embodiment of the electric circuit ac cording to theinvention as shown in Figure 4 a resistance M of the order of a few ohmsis inserted in series with a permanent tap S from the regulatingtransformer A which is connected with the primary windings of both thehigh voltage transformers and the filament current.

transformer. In the same way as described above the increasedconsumptionby adjustment of a higher anode Figure 5 shows a fifthembodiment of the electric. circuit according to the invention, in whichthe anode voltage is adjusted by means of a double switch 02, so thatsimultaneous with the increase in the anode voltage the voltage acrossthe combined cathode transformer and filament current transformer F isreduced, wherefore the temperature and emission of the filament willdecrease so that the increase in the anode current that would haveoccurred owing to the increased anode voltage is equalised.

*Figure 6 shows a sixth embodiment of the electric circuit according tothe invention, incorporating an anode transformer and a combinedcathode/filament current transformer F. A compensation transformer Hdimensioned for a few volt-amps has its primary winding parallel withthe primary winding of the anode transformer B, whereas its secondarywinding is in series with the primary winding of the cathode andfilament current transformer F, but counter-connected to the same. Itwill be evident from this that on an increase in the voltage of theanode transformer H a voltage will be subtracted from the cathode andfilament current transformer F, whereby the temperature and emission ofthe filament will decrease so that the increase in the anode currentthat would have occurred owing to the increased anode voltage isequalised.

Figure 7 shows a seventh embodiment of the electric circuit according tothe invention in which the primary winding of the auxiliary transformerH is connected in parallel with the primary windings of the high voltagetransformers B and C, whereas its secondary winding is in series withthe primary winding of the filament current transformer D, butcounter-connected to same. It will be evident from this that by anincrease in the voltage of the anode transformer B a voltage will besubtracted from the filament current transformer, whereby thetemperature and emission of the filament will decrease so that theincrease in the anode current that would have occurred owing to theincreased anode voltage is equalised.

The embodiments of the electric circuit according to the invention asdisclosed in Figures 3-7 are based on the same principles as theembodiments of the invention disclosed in Figures 1 and 2, the internalvoltage of the apparatus being controlled by means of the milliammeter mand adjusted by means of the contact brush K for adjustment of the inputvoltage of the apparatus.

a-selected value of the anode current'and of-the kind comprising atleast one high voltage transformer, between whose secondary windings isinserted a milliammeter and a filament current transformer which may beformed as a special winding carried on one of the high voltagetransformers, whose primary windings across adjustable taps areregulably connected to a supply transformer for instance anautotransformer, which is connected to a nominal supply voltage througha regulating device for equalizing variations in the said nominal supplyvoltage, characterized in that'the primary winding of the filamentcurrent transformer or the primary winding of the highvolt-agetransformer carrying said special winding is directly connectedto fixed taps so arranged on said autotransformer that when theautotransformer is adjusted to the nominal supply voltage the anodecurrent is equal to the substantially constant anode saturation currentof the tube.

2'; An electric circuit as'claimedin claim 1, including a; device tocompensatefor the increase, if any, in the saturation current of theX-raytube at increasing anode voltage, characterized in that the saiddevice consists of a fixed resistor inserted in one of the supply linesfrom the supply voltage to the autotransforrner and so dimensioned thatthe voltage drop in the resistor compensates for a small increase in theanode saturation current, which increase is substantially proportionalwith the anode voltage. I

3. An electric circuit as claimed in claim 2, characterized in that thedevice consists of a fixed resistor inserted in one of the supply linesto the primary winding of the filament current transformer or of thehigh voltage transformer carrying said special winding and sodimensioned" that the*v'o'lt'a'ge drop in the said. resistors:compensates for a small increase in the anode=saturationcurrentwhichincrease is substantially proportionalwitlr';

the anode voltage.

manner that the filament current decreases by moving said switches inpredetermined manner at increasing anode voltage. I

5. An electric circuit as claimed in claim 1, characterized in that acompensation transformer whose primary winding is connected in parallelwith the primary winding of the anode transformer, whereas its secondarywinding is counter-connected in series with the primary winding of thehigh voltage transformer having the special filament current winding.

' 6. An electric circuit as claimed in claim 5, characterized insaid'compensation transformer having its primarywinding connected inparallel with the primary windings of'the high voltage transformers,whereas its secondary winding is counter-connected in series with theprimary winding of the filament current transformer;

References Cited in the file of this patent UNITED STATES PATENTS1,631,625 Coolidge June 7, 1927'" 2,167,802 Goldfield Aug. 1, 19392,172,581 Horsley Sept. 12, 1939' 2,492,281 Hall Dec. 27, 1949 2,626,360Wright Jan. 20, 1953 2,745,020 Graves et al. May 8, 1956' 2,870,340Fransen Jan. 20, 1959'

